Polymer compositions are used for a plurality of technical applications. For electrical applications, the bulk and surface resistivity of a polymeric composition can vary over many orders of magnitude from being highly electrically conducting to being highly electrically insulating. This variation in electrical resistivity is generally dependent upon the amount of electrically conducting filler contained in the polymeric composition.
Changing the temperature can also bring about a change in the electrical resistivity of an electrically conducting polymer composition. For example, at low temperatures, the resistance of the electrically conducting polymer composition is low and allows a large amount of electrical current to flow through the polymeric composition. As the temperature is increased up to a point, there is an increase in the electrical resistivity of the polymeric composition. A function/curve of the electrical resistivity with temperature has a positive slope and within this temperature range, the electrically conducting polymer composition is said to have a positive temperature coefficient resistance (PTCR).
According to one theory, it is believed that a mismatch between the coefficient of thermal expansion between the polymer and electrically conducting filler causes a localized disruption in the electrically conducting network that leads to an increase in the electrical resistivity of the composition. In most cases, the required thermal expansion is caused by a phase transition, such as melting, of the polymer leading to the PTCR near the trip temperature. This however often leads to deterioration of mechanical properties of the composition near the trip temperature, as for example, one would expect from melting of the polymer matrix. It is therefore beneficial for the electrically conducting polymer composition to display the PTCR trip behavior below a temperature where no such deterioration of mechanical properties takes place. In still another theory, the physical and electrical properties of the electrically conducting filler are believed to change with temperature causing a change in the electrical resistivity of the polymeric composition.
As the temperature is raised further, however, often the electrical resistivity of the polymeric composition drops with temperature. The electrically conducting polymer composition now displays a negative temperature coefficient of resistance (NTCR). This change from PTCR behavior to a strong NTCR behavior is often undesirable. There have been several theories put forth to explain this behavior.
While the aforementioned theories purport to explain this behavior in the electrical resistivity, there are some applications for which such behavior is undesirable. It is therefore beneficial to have polymer compositions for certain applications where the change in behavior from a PTCR polymeric composition to a NTCR polymeric composition is minimized. In other words, it is beneficial for the electrically conducting polymer composition to display only PTCR behavior over the temperature range of the intended application.
To get positive temperature coefficient to resistivity (PTCR) effect, the composite material should be conductive. In order to make a composition conductive, the prior art added conductive filler particles above the percolation threshold. The percolation threshold is the point at which the formation of long-range connectivity in random systems. Below the percolation threshold, a composite will be non-conductive. However, loading of high percentage of high specific gravity fillers, like metal, increases the specific gravity of the composite considerably.
Accordingly, it would be beneficial to provide a way of reducing the specific gravity of the composition and increasing the performance to cost ratio by adding low specific gravity second fillers in tuning the trip characteristics of PTCR formulation.